1. Field of the Invention
This invention relates to computer graphics systems and more particularly to the rotation of video display images in video display systems employing raster-scan display techniques.
2. Description of the Relevant Art
Most computer systems use video display systems which employ the same raster-scan display techniques used in television sets. Digital video information stored in a memory system is used by a video display system to create an image on the screen of a display device such as a cathode ray tube, a flat-panel display, or a liquid crystal display.
Video display systems employing raster-scan display techniques typically store images as horizontal rows of their component points, or pixels. Images are "painted" on a screen of a display device point by point, line by line, from the top of the screen to the bottom. The need to recreate images on the screen of a display device many times each second is a characteristic many video display systems employing raster-scan techniques share. The light output of a cathode ray tube fades quickly, hence a video display system with a cathode ray tube display device must refresh an image many times each second to avoid flicker. An image created on a flat-panel display must be refreshed many times each second in order to produce varying shades of gray, or to produce a color scale if the display device is capable of displaying color information.
Pixel information corresponding to each point on a screen of a display device is typically stored in a memory element called a frame buffer. This pixel information includes the intensity (i.e., brightness) of a point to be displayed, and may also include color information if the display device has the ability to display colors. Most frame buffers are comprised of a large number of storage locations in a semiconductor memory system. Indeed, it was the availability of relatively inexpensive semiconductor memory systems which made raster-scan display systems cost effective. Pixel information is typically stored in a frame buffer in the order in which each pixel is to be displayed on a screen of a display device. The number of pixels per horizontal scan line and the number of horizontal scan lines per display screen determine the resolution of a raster-scan display system.
A block diagram of a typical raster-scan video display system is shown in FIG. 1. A CPU is coupled to a system bus, and a video controller is also coupled to the system bus. A display device is coupled to the video controller. The CPU sends video information to the video controller in digital form via the system bus. The video controller converts this digital video information to signals used by the display device to display raster-scan images.
Turning now to FIG. 2, a block diagram of a conventional video controller as provided in FIG. 1 is shown. The video controller comprises a bus interface unit, a frame buffer, and an image display system. The bus interface unit is coupled to the system bus and to the frame buffer. The frame buffer is coupled to the image display system. The image display system is coupled to the display device. The bus interface unit serves as the interface between the system bus and the video controller. Video information is stored in the frame buffer in digital form. Each memory location of the frame buffer contains the information associated with one or more pixels to be displayed. The image display system fetches digital pixel information from the frame buffer, converts the digital information to signals the display device can use, and provides these signals to the display device. The image display system may cycle through the digital pixel information stored in the frame buffer many times each second to recreate the image on the screen of the display device. Frame buffer memory addresses may be generated in synchronism with the raster scan of the display device, and the contents of each memory location in the frame buffer may be used to control the image display process.